It is desirable for tires to have good wet skid resistance, low rolling resistance, tear strength, and good wear characteristics. It has traditionally been difficult to improve the wear characteristics of a tire without sacrificing wet skid resistance and traction characteristics. These properties depend, to a great extent, on the dynamic viscoelastic properties of the rubbers utilized in making the tire. Such viscoelastic properties are controlled largely by the microstructure of the elastomer.
In order to reduce the rolling resistance and to improve the treadwear characteristics of tires, rubbers having a high rebound have traditionally been utilized in making tire tread rubber compounds. On the other hand, in order to increase the wet skid resistance of a tire, rubbers that undergo a large energy loss have generally been utilized in the tread of the tire. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubbery material for automobile tire treads.
It has been generally conventionally believed to be desirable, e.g., for styrene-butadiene rubber, when utilized in tire tread compounds, to have a high level of vinyl content (1,2-microstructure). To this end, styrene-butadiene rubbers are often synthesized by solution polymerization that is conducted in the presence of a catalyst system, including one or more modifying agents. In contrast to providing high vinyl content rubbery polymers, it is believed that rubbery polymers having high trans microstructure contents may provide a more desirable balance of tire tread properties.
Thus, it is desirable to provide catalyst systems for synthesizing rubbery polymers, such as polybutadiene rubber and styrene-butadiene rubber, having a high trans microstructure content, wherein the rubbery polymers can provide desirable wear properties without substantially sacrificing other performance characteristic(s), e.g., traction properties.